The present invention relates to transmissions of the multiple, substantially identical countershaft type, wherein means are provided for ensuring an even distribution of torque on all of said countershafts. More particularly, the present invention relates to such a transmission of the type having power take-off capability.
Although the present invention may be utilized in various types of change gear transmissions, it is especially suited for use in twin countershaft transmissions designed primarily for heavy-duty truck applications, and will be described in connection therewith. Twin countershaft transmissions of the type to which the present invention relates are illustrated and described in detail in U.S. Pat. No. 3,105,395, assigned to the assignee of the present invention and incorporated herein by reference.
The use of at least two substantially identical countershafts has a number of advantages which have been recognized by those skilled in the art at least since the issuance of the above-incorporated patent. The primary objective of a twin countershaft transmission is to ensure that the multiple countershafts each carry an equal share of the total torque being transmitted from the input shaft of the transmission to the output. If the torques are not substantially evenly divided between the countershafts, the true purpose of providing multiple countershafts is not achieved.
In various vehicle applications for transmissions of the type to which this invention relates, it is desirable to be able to direct all or a portion of the output power from the vehicle engine, through the transmission, to an auxiliary device. As is well known to those skilled in the art, an arrangement for taking off power from a transmission for an auxiliary device is typically referred to as a "PTO" (Power Take-Off). An example of such a vehicle application requiring a PTO would be a truck in which the trailer is equipped with a "moving floor" to facilitate loading or unloading of the contents of the trailer. Other examples would include a cement mixer, in which the drum must be rotatably driven, or the pumper on a fire truck, wherein the PTO drives the water pump.
In a typical, prior art PTO arrangement for twin countershaft transmissions, one or both of the countershafts is provided with a PTO gear, and an adjacent portion of the housing defines a window. An auxiliary device may be mounted to the transmission housing, at the window, with the device including a gear engageable with the PTO gear on the countershaft. The output of the gear in the PTO device may then be used to provide a mechanical PTO, or the gear may be used to drive a pump for a fluid pressure actuated (hydraulic) PTO.
The primary disadvantage of the typical, prior art PTO arrangement described above, whether mechanical or hydraulic, is that, by taking the PTO power from a single countershaft, the principle that power (or torque) be shared equally by countershafts is violated. One result of such an arrangement can be an "imbalance" whereby the gears on the main shaft are not balanced in the desired manner, as they normally are when power is being shared equally by the countershafts. This is especially undesirable in situations where the PTO must be useable while the vehicle is being driven, such as in the case of the cement mixer wherein the drum must be rotated while the mixer is driven to the job site.